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Moving algae the key to renewable oil production?

An array of transparent cylinders glow jade in the dark laboratory, a brilliant green elixir in bubbling columns. The lab is quiet except for the bubbling of the liquid that gently froths at the top of the glass cylinder. 

There are 15 of these torso-long cylinders, each suspended vertically on a steel frame. Thin black tubes running underneath like cables connect the cylinders to an automated control panel that measures the precise amount of air and carbon dioxide sent to each. Fluorescent lights shine from behind the row of tanks giving them an ethereal glow. These algae growth tanks are known as airlift photobioreactors, a high-tech tool for producing renewable energy products from algae. 

Benefits

The use of algae as food, animal feed, fertilizer and medicine are ancient concepts, but with today’s global energy concerns there is renewed interest in algal biotechnology, with algal oil being a possible weapon in the battle against climate change.

Algal oil has already reached commercial-scale production in the United States, where the company Sapphire Energy produces “Green Crude”, later refined to gasoline (what we call petrol), diesel or jet fuel. Green Crude, developed from micro-algae grown in large, shallow open-air ponds, was sold to its first commercial customer, a transportation fuels corporation and Fortune 100 company called Tesero, in 2013. 

This production method is popular and can be low cost, but Yusuf Chisti, a professor of biochemical engineering at Massey University in New Zealand, has shown that photobioreactors produce algae faster than ponds because the reactors are less susceptible to contamination and weather changes. 

Unfortunately, photobioreactors tend to be energy intensive, leading to increased running costs and a larger carbon footprint. If the energy required to grow the algae is greater than the energy harnessed from the algal oil, then the process is neither economically nor environmentally feasible. 

At the University of Cape Town’s Centre for Bioprocess Engineering, 12 researchers are researching how to improve these bubbling green vessels and make the algae grown in them more appropriate for oil production. One way is the energy used to “mix” it (run bubbles through the algae). 

Algae need to be mixed to be healthy, so that the light, carbon dioxide and other essential nutrients are evenly circulated to the single-celled organisms in the vessel. This is unfortunately quite energy intensive due to the electrical power required to supply compressed gas for bubbling. 

One of the group’s studies includes two photobioreactor designs that are different from the airlift photobioreactor.  The first is a transparent plastic bag, inflated with air (like an oval bubble) and fastened onto a rocking platform. The liquid algae fills half of the bag and the rocking motion creates a sloshing wave. 

The second is a horizontal cylinder containing oscillating half-moon shaped paddles. As the paddles move back and forth, the liquid algae flows over the top of the paddles. Both of these designs require less bubbling of compressed gas, because the motions of the waves and oscillations keep the nutrients circulating. We believe that these alternate ways of mixing will prove more energy efficient.  

Another way to improve efficiency is to increase the amount of oil that the algae produce. We specifically chose a common freshwater micro-algae species found in ponds and rivers globally, called Scenedesmus, which produces large quantities of oil. We can also alter the nutrient mix so that the biological processes in the algae favour oil production rather than cell growth. Additionally, we are also trying to pinpoint the energy-input-to-productivity sweet spot, the point at which oil production is the most efficient. 

Energy efficiency

The first peer-reviewed output from this work, published in March this year in journal Algal Research, explores the bubble rate and carbon dioxide ratio in the airlift photobioreactor. The airlift photobioreactor is often used for algal production but this is the first time its energy efficiency has been tested by simultaneously varying the gas bubbling rates and the carbon dioxide concentrations. The data shows that the bubbling rate can be decreased to a critical level without affecting the algae, as long as carbon dioxide concentrations are sufficient. This means that we can greatly reduce the energy required for algal production, without compromising the quantity of oil.

In this time of environmental instability and strained resources, will algal oil become an important part of our future, and a key player in the shift to renewable energy sources? 

From the lab, where scientists tinker busily with a rack of bubbling and sloshing tanks, to the vast commercial algae ponds spreading across hundreds of acres, algal biotechnologists around the world have great hopes for the future of algal oil and continue to search for ways to improve energy efficiency and make production feasible. 

Sarah Jones is a PhD candidate at the University of Cape Town.

This publication is the culmination of a six-month-long Mail & Guardian project, called Science Voices, to teach postgraduate science students how to turn their academic writing into something the public can read and enjoy.

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